[755-1] Hierarchical Logic Designs

[Konstantinos Dovrolis <dovrolis@cs.wisc.edu>]

This is not really a FAQ, but it addresses an interesting issue:

"
Hi
The thing which is confusing me is following.
1)I make a block using da that takes one bit of xi, yi and ci and
generates sum and carryout bit.

2) Using block obtained in 1, I make blocks for 4bit, 5bit and 6bit.

3)Now I open new schematic and I use the blocks which I made in step2 to
get the schematic for 19bit Carry select adder. 

4)Now in IC station how can I specify that those blocks of 4bit, 5bit and
6bit used in 19bit CSL design should be placed in layout. Or should I
don't worry about it and do auto place and route in this assignment.
"

Essentially, the question is: If we have a certain hierarchy
in the logic design, does this hierarchy map to the layout generation?

The answer is "It depends". You can either totally ignore the logic
design hierarchy when you go to IC station, creating in that way what
is called "a Flat layout". This is the procedure that tutorial-4 explains. 
The point that you to note here is that the IC Blocks viewpoint has
to be created on the top-level design (in this case, the 19-bit adder).

Alternatively, you can map the hierarchy of the logic design (or
any other hierarchy you want) to the layout. The basic idea in that
case is that you will create different pieces of layout for different
logic designs, and in the top level of the layout hierarchy you will
instantiate and connect (manually or automatically) these pieces
together. You will have to do this for your final project.

There are several more things that need to be said here, but it is 
better if we discuss them tomorrow at the discussion session.

C.